This invention relates to the identification of failed fuel elements in a fast breeder reactor. The invention also relates to a method of operating a fast breeder reactor with failed fuel elements therein.
Conventionally, the gross radioactivity of a reactor coolant and/or the off-gases from the reactor are monitored during operation. An abrupt rise in radioactivity indicates that a cladding leak has occurred somewhere in the system. Experimental reactors are thereupon shut down and the offending element is removed from the system and replaced. Operation of power reactors is usually continued with the leaker present -- frequently with a number of leakers present -- either until a scheduled shutdown occurs or until the gross radioactivity of the coolant and off-gas becomes too high to tolerate. After shutdown all offending elements are removed and replaced.
In both cases identification of the failed fuel elements is necessary. Attempts have been made in the past to monitor the coolant exiting each subassembly in the reactor to determine directly the location of the failed fuel element. Unfortunately, success in this can only be attained with equipment which is extremely complex mechanically.
Identification is accomplished in most commercial light-water reactors by a technique known as "sipping". Following shutdown of the reactor the coolant in each subassembly is sampled and the sample is checked for certain radioactive fission products.
Tagging of subassemblies with a unique identifiable mixture of radioactive isotopes has also been suggested and tagging of fuel subassemblies placed in EBR-II (The Experimental Breeder Reactor #2) with mixtures of xenon isotopes having a mass number of less than 131 is now being accomplished on a routine basis. Such a procedure, unfortunately, is very expensive and it is estimated that it will cost $400,000 to xenon tag the fuel subassemblies for the Fast Test Reactor included in the Fast Flux Test Facility now under construction at Richland, Wash. In addition, xenon tagging is not wholly satisfactory since the tag ratios change with irradiation. A particular problem in EBR-II is that the older experimental subassemblies in that reactor are not xenon tagged and it is these subassemblies that are most likely to leak. Thus, there is a continuing need for new procedures for identifying failed fuel elements and the need is expected to increase tremendously in the future as fast breeder power reactors are put on the line.